But the insects of Celebes present us with other phenomena more curious and more difficult to explain than their striking individuality. The butterflies of that island are in many cases characterised by a peculiarity of outline, which distinguishes them at a glance from those of any other part of the world. It is most strongly manifested in the Papilios and the Pieridae, and consists in the forewings being either strongly curved or abruptly bent near the base, or in the extremity being elongated and often somewhat hooked. Out of the 14 species of Papilio in Celebes, 13 exhibit this peculiarity in a greater or less degree, when compared with the most nearly allied species of the surrounding islands. Ten species of Pieridae have the same character, and in four or five of the Nymphalidae it is also very distinctly marked. In almost every case, the species found in Celebes are much larger than thane of the islands westward, and at least equal to those of the Moluccas, or even larger. The difference of form is, however, the most remarkable feature, as it is altogether a new thing for a whole set of species in one country to differ in exactly the same way from the corresponding sets in all the surrounding countries; and it is so well marked, that without looking at the details of colouring, most Celebes Papilios and many Pieridae, can be at once distinguished from those of other islands by their form alone.
The outside figure of each pair here given, shows the exact size and form of the fore-wing in a butterfly of Celebes, while the inner one represents the most closely allied species from one of the adjacent islands. Figure 1 shows the strongly curved margin of the Celebes species, Papilio gigon, compared with the much straighter margin of Papilio demolion from Singapore and Java.
Figure 2 shows the abrupt bend over the base of the wing in Papilio miletus of Celebes, compared with the slight curvature in the common Papilio sarpedon, which has almost exactly the same form from India to New Guinea and Australia. Figure 3 shows the elongated wing of Tachyris zarinda, a native of Celebes, compared with the much shorter wing of Tachyris nero, a very closely allied species found in all the western islands. The difference of form is in each case sufficiently obvious, but when the insects themselves are compared, it is much more striking than in these partial outlines.
From the analogy of birds, we should suppose that the pointed wing gave increased rapidity of flight, since it is a character of terns, swallows, falcons, and of the swift-flying pigeons. Ashort and rounded wing, on the other hand, always accompanies a more feeble or more laborious flight, and one much less under command. We might suppose, therefore, that the butterflies which possess this peculiar form were better able to escape pursuit.
But there seems no unusual abundance of insectivorous birds to render this necessary; and as we cannot believe that such a curious peculiarity is without meaning, it seems probable that it is the result of a former condition of things, when the island possessed a much richer fauna, the relics of which we see in the isolated birds and Mammalia now inhabiting it; and when the abundance of insectivorous creatures rendered some unusual means of escape a necessity for the large-winged and showy butterflies.
It is some confirmation of this view, that neither the very small nor the very obscurely coloured groups of butterflies have elongated wings, nor is any modification perceptible in those strong-winged groups which already possess great strength and rapidity of flight. These were already sufficiently protected from their enemies, and did not require increased power of escaping from them. It is not at all clear what effect the peculiar curvature of the wings has in modifying flight.